It is known that changes in position on the part of patients with cardiac pacemakers frequently give rise to inappropriate or disproportionate reactions from the pacemaker. One reason for this is that the change in position gives rise to considerable variations in parameters in the circulation of the blood, more particularly because of the altered influence of the force of gravity. Frequently, a variation in intercardial or transthoracal impedance, caused by a change in position, is misinterpreted by a control system of the pacemaker as physical activity, to which the pacemaker reacts with a change in the heart rate which is inappropriate in relation to the change in position. That gives rise to problems. Admittedly, an inappropriate variation in heart rate of that kind is generally not life-threatening, but it is perceptible for the patient. In particular a patient who is lying down and who merely changes his position from lying on the right side to the left side will experience changes in heart rate which are caused thereby as being uncomfortable. A more serious consideration is that, in the case of self-adapting pacemakers, such unnecessary changes in heart rate can have an adverse influence on the adaptation performance of the pacemaker, with the result that the pacemaker then no longer adequately reacts to physical loads and stresses.
In order to avoid that, pacemakers are provided with detectors for detecting changes in position. U.S. Pat. No. 5,593,431, to Sheldon, discloses a pacemaker with an implanted accelerometer as a position detector. The accelerometer which acts in three axes measures the direction of the pull of gravity and determines therefrom the position of the patient. As however implants generally have a tendency to turn within the body of the patient, there is the risk of the reference system of the accelerometer suffering from displacement. That makes gravitational measurement unreliable and position detection uncertain. It is also known for signals of an accelerometer to be subjected to frequency analysis in order to determine therefrom rest and activity phases, as disclosed in Thompson, U.S. Pat. No. 5,233,984. A disadvantage with that method is that a change between a rest phase and an activity phase does not necessarily correlate with a change in position. That gives rise to detection errors, resulting in an only low level of accuracy in terms of position sensing.
The object of the present invention is to provide a pacemaker which has a reliable detection device for changes in position, which enjoys long-term stability.
In accordance with the invention that object is attained by a pacemaker having the features of claim 1. Advantageous developments are set forth in the appendant claims.
In accordance with the invention, in an implantable cardiac pacemaker comprising a control device including a position detector connected to a movement sensor, it is provided that the position detector has a classification device for the recognition of short movements. The invention is based on the notion that a short signal from the movement sensor is a standard or benchmark for a change in position. It accordingly turns away from the approach followed in the state of the art, namely, ascertaining changes in position by interpretation of contents of the signals from the movement sensor. This is based on the realization that it is crucially important to ascertain the change between different positions of the body, and that it is possible to forego determining the absolute position of the body of the patient. The invention realized that classification in accordance with the duration of the signal from the movement sensor can be used not only for reliably detecting changes in position but in addition also advantageously for determining the hemodynamic relevance of the change in position. This is based on the realization that a change in position is all the more significant in terms of the hemodynamics of the circulation of the blood, the more rapidly the change in position takes place; conversely, a change in position which takes place slowly is only of slight hemodynamic relevance.
The pacemaker according to the invention also has the advantage that it is not really expensive. The movement sensor can be a simple known activity sensor, an expensive multi-axis accelerometer is not required; linearity and sensitivity are only of subordinate significance. As the movement sensor of the pacemaker according to the invention, in contrast to the three-axis accelerometer, does not require a given reference system, the pacemaker according to the invention is robust in relation to the implant turning, thereby improving long-term reliability.
In order to be able to determine the duration of a signal from the movement sensor the classification device desirably has a short-term counter and a switch device. The term counter in accordance with the present invention is to be interpreted broadly; it includes both event counters and also time counters. Admittedly, counters are usually adapted to operate in a discrete manner, but counters which operate continuously should not be excluded. An event counter which is adapted to count clocks while a movement signal is applied is particularly advantageous. As a clock signal is usually already present in pacemakers for a microprocessor, the additional expenditure required for the counter is thus slight.
Desirably the comparison device is such that at the end of the movement signal it compares a counter state to a predeterminable threshold value and outputs a position change signal. The predeterminable threshold value makes it possible to achieve a patient-specific setting for the time duration, up to which a signal from the movement sensor is to be detected as a change in position. In that way the pacemaker according to the invention can be matched to the hemodynamics of the respective patient. The switch device is possibly also connected to a telemetry device of the pacemaker; that makes it possible to modify the threshold value from the exterior even after implantation. At its output the switch device provides a position change signal which can assume various states and which is an input signal for other function modules of the control device.
Advantageously the position detector has a memory device. The memory device stores past movements, in which respect the memory device has only a limited depth of recollection. The memory device represents an indicator as to whether prior to a movement to be classified other movements have already taken place, that is to say whether an activity state is involved, or whether previously there was a rest phase, that is to say a passivity state is involved. As a change in position from a passivity state is hemodynamically more relevant than one from an activity state, the memory device is advantageous in terms of evaluating the hemodynamic relevance of a change in position. The position detector is therefore not limited to signals which are supplied at the present time by the sensor, but it can additionally be adapted to evaluate signals in terms of preceding movements; that permits more accurate recognition of changes in position. The memory device is connected to the switch device; the switch device is designed in such a way that, in a corresponding manner as in the case of the short-term counter, at the end of a movement signal, it compares the value of the memory device to a predeterminable second threshold value. The predeterminable second threshold value makes it possible to achieve a patient-specific setting for the memory device. Desirably, the switch device is designed for interlinking the comparison results of the short-term counter and the memory device, in such a way that, for the output of a position change signal, the movement signal must be correspondingly shorter, the higher the activity state. The cardiac pacemaker then involves a physiologically correct behavior, whereby a change in position correspondingly more requires an appropriate reaction on the part of the pacemaker, the correspondingly less activity or even no activity at all has preceded, and vice-versa.
Desirably, the memory device has an integrator. The integrator is adapted to add periods of time, during which a signal of the movement sensor is applied and for subtracting periods of time during which no signal from the movement sensor is applied. It thus acts as a memory for preceding movements. An adjustable maximum in respect of the value of the integrator limits the depth of recollection.
In a preferred embodiment, the short-term counter and the integrator are integrated into a counter. Besides a reduced level of expenditure, that affords the advantage that only one threshold value needs to be set and nonetheless the physiologically correct interaction between preceding activity and duration of the movement signal is retained.
Desirably, there is provided an elapsed time counter, at the input of which the position change signal is applied and at the output of which a blocking signal is outputted. Such a counter provides an information signal, as to how long the position change signal is already applied, and thus makes it possible to determine the time which has passed since the detected change in position. That makes it possible for the control device to react in time-matched relationship to a change in position, for example to limit its reaction to a given period of time.
Desirably there is provided a cancellation or clearing device for the blocking signal, the triggering device thereof being connected to the movement sensor. The cancellation or clearing device provides that, upon the occurrence of a fresh movement, the position change signal is canceled or cleared, and the pacemaker can react to the fresh movement, without being adversely affected by the position change signal.
Preferably, the control device has a position-dependent heart rate limiter. That can provide that the stimulation frequency does not rise above a given limit value, in the event of movements of the patient which are due to changes in position. Desirably, a position change signal and a heart rate signal are applied to the heart rate limiter. The heart rate signal is a signal calculated by the control device, for exciting the cardiac muscle. In dependence on the position change signal, the heart rate limiter limits the calculated heart rate signal by means of an adjustable limiting function and outputs it. The term limiting function is used to mean a function whose output value rises less than its input value. The heart rate limiter can be such that, at a value in respect of the calculated heart rate signal which is higher than a limit value, the outputted heart rate signal is of precisely that limit value; that is referred to as rigid limitation. Advantageously however the heart rate limiter is adapted to afford a flexible limitation effect, such that, at a value of the calculated heart rates, which is higher than a limit value, the value of the outputted heart rate is between the calculated heart rate and the limit value.
The control device advantageously has an inhibitor. The inhibitor serves to prevent adaptation of parameters of the control device, induced by a change in position. That ensures that the parameters are not altered by changes in position in such a way that the frequency-adaptive behavior of the pacemaker is worsened. For that purpose, it is desirably provided that the inhibitor is cooperatively connected to an adaptation module in such a way that adaptation is blocked in position-dependent manner. That avoids self-adapting parameters of the pacemaker being influenced during the application of the position change signal, as otherwise the behavior of the pacemaker could be worsened in a situation involving a physical load.
In an alternative embodiment the implantable pacemaker is connected to a sensor for the physiological demand, in particular an impedance sensor for measuring intercardial or transthoracal impedance. In addition, this pacemaker has control means for setting a stimulation rate or heart rate in dependence on a signal coming from the physiological demand sensor. In that case, control of the heart rate can be limited in such a way that the changes in rate as a result of the physiological demand are limited in respect of their magnitude in particular upwardly but also downwardly. That therefore affords a band of permitted heart rates. In the alternative pacemaker the position of that band, that is to say the position of the upper and lower rate limits, is determined by a signal from the movement sensor. In that way it is possible to achieve a rate limitation effect in the case of a change in position, just as an increased stimulation rate can be permitted in the event of continuous movement and increased metabolic demand.